The present invention concerns a substrate coating that is dust-repellent and is easily cleaned by rain washing off adhering inorganic and organic dirt. The coating imparts to the glass in the preferred embodiments additionally an antireflective property and is suitable therefore in particular for coating glass that is used in devices for solar energy utilization, for example, as plate glass for covering photovoltaic panels and water heating collectors or tubular glass for solar water heating but also optionally for construction glass or the like. For producing the coating, the substrate to be coated is immersed in a coating solution and is then pulled from the solution at an appropriate speed. The wet film that remains on the glass surface is dried and subsequently, as needed, is baked at several hundred degrees, for example, 500° C., for increasing the wear resistance. The technical device or other substrate, furnished with a glass that is finished in this way is suitable primarily in geographical regions in which a lot of dust is present in the atmosphere and, as a result, the energy yield is reduced by dusty irradiation surfaces.
Components of our atmosphere encompass very small gas molecules with diameters below 1 nanometer up to ash particles that have a size of up to 100 μm. The group of aerosols are solid or liquid particles in the air with a typical size between 0.01 and 10 μm as well as—in contrast to greenhouse gases—a very short life span of a few days. Aerosols can be of natural and/or anthropogenic origin (sea salt, mineral dust, sulfate, soot etc.). The particulate materials that are disadvantageous for effective utilization of solar devices are primarily dust with inorganic and organic components that is sparingly soluble in water, or soot or smoke that are primarily comprised of carbon. The smallest of these aerosols have a diameter of 10 nm (ultra-fine dust) that does not form a sediment but instead is deposited as a much larger coagulate of about 100 nm in size by precipitation. Sediment dust has a diameter in the micrometer range, by definition more than 10 μm.
When aerosols are deposited on the glass surface, the transmission is reduced.
Particularly damaging in this connection are particles that collect as a permanent dirt layer on the glass and cannot be removed by rain or wind and require a mechanical cleaning action (for example, with a sponge and brush).
Examinations of soiled glass surfaces of a collector have shown that a dust cover layer is positioned on a smeary base layer that is adhering well. The dust cover layer may be removed by raindrops but not the base layer. It is in particular this permanently acting soiling that impairs the effective utilization of solar devices.
On soiled glass tubes the chemical composition of the surface was analyzed by means of TOF-SIMS methods (time-of-flight secondary ion mass spectroscopy; time-of-flight SIMS). In high intensity Na, K, Mg, Ca, Fe, Al, Si and Pb have been found. In addition to Cl and NO, that form water-soluble compounds, also F and SO have been detected that both may be sparingly soluble when present as MgF2, CaF2 or gypsum. Moreover, organic compounds, including higher fatty acids, are present that are also sparingly soluble and that may explain the smeary, oily consistency of the permanent base layer.
As already explained in DE 103 51 467 A1 it is known to provide articles either with hydrophobic solutions or with hydrophilic photocatalytic layers for avoiding dirt depositions. Hydrophobic surfaces can be generated in various ways. On the one hand, as is conventional in the ceramics industry, a layer that is comprised of an inorganic-organic nano particle network can be crosslinked by means of unsaturated organic groups thermally or by UV light. Typical examples are the inorganic-organic hybrid polymers that are known by the trademark ORMOCER® of the Fraunhofer-Institut für Silicafforschung. On the other hand, there is a plurality of hydrophobic, organic solutions that can be applied after manufacture or even by the customer (for example, solutions that have become known by the trademark “Clear Shield™”). WO 00/37374 illustrates an example of such an approach. The coatings described in this publication are however mechanically not very stable and are also limited, as a result of the organic components, with regard to maximum temperature of use. Moreover, façade glasses are offered with easily cleanable hydrophilic photocatalytic layers. In order to achieve a cleaning effect, activation by UV light is however mandatory. Moreover, the decomposition rate is very low and is not suitable for contact soiling.
WO 03/027015 A1 discloses a hybrid sol which in addition to water and solvent contains two different fractions of [SiOx(OH)y]-particles with 0<y<4 and 0<x<2 wherein the first fraction has a particle size of 4-15 nm and the second fraction an average particle size of 20-60 nm. The sol may be obtained by hydrolytic polycondensation of tetraalkoxysilanes in an aqueous medium containing a solvent wherein silicon oxide hydroxide particles are obtained. A monodisperse silicon oxide hydroxide sol with an average particle size of 20-60 nm is added to the medium when the hydrolytic condensation of the silanes has occurred at least partially. The hybrid sol is suitable for producing wear-resistant SiO2 antireflective layers on substrates, preferably on glass, that are particularly suitable for covers of solar collectors and photovoltaic cells. WO 03/027034 A2 discloses antireflective surface coatings produced based on such sols with a ratio of particles of the first fraction to the particles of the second fraction in the range of 3,000:1 to 250:1. The degree to which these coatings can be dust-repellent or aerosol-repellent is not mentioned in the cited publications. Based on information of those inventors of the present invention that are also inventors of the coatings disclosed in the aforementioned WO publication, in field tests no significant differences with regard to dust deposition between uncoated and coated glass was found.